Appreciate the links and ideas, guys. Well, a few months ago, I had the
manifold off as there were some issues with the pcv valve and removing
the manifold was the only way to get to it. Since I had it off, this is
the 2nd time the MIL has come on. The first one was a code for one of
the O2 sensors and that turned out to be a connection that I forgot to
hook back up after replacing the manifold. With this problem, my first
thought was maybe a loose or intermittent connection as after all the
plastic connectors are now quite brittle with a car this old, so I
pulled the connectors leading to the DPFE, cleaned both sides of the
connection with contact cleaner spray, than then plugged them closed
again. Driving cycle earlier did not cause the MIL to come on, so will
now wait and see over the coming days whether it comes back. I
understand that this code sometimes needs several driving cycles to
appear. Hope not :(

I had PO401 code (insufficient EGR flow), isolated to the DPFE and
replaced it Tuesday, drove about 20 minutes total, stopping at 2-3
stores Tuesday night, and the check engine light went off without having
to reset via OBD2. Smog check passed on Wednesday with flying colors.

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